Here's a pretty amazing machine that
brought forth (with me, at least) the realization that Monroe not only resold
calculators designed and manufactured by Canon and Computer Design Corporation,
they also resold early electronic calculators made by the West German company
Olympia. The Monroe 740 is a re-badged (with subtle cosmetic changes) version
of an early Olympia calculator. The machine is built to the typically
high quality levels of German engineering -- a precision instrument built
with extreme craftsmanship.

Profile view of the Monroe 740

The 740 appears to be the first generation
of electronic calculators that Monroe resold in the US. It is an
all-transistor machine, built in the mid-1968 timeframe. The machine uses
magnetic core memory for storage of working registers, and classic Nixie
tube displays. Though this particular machine appears to have been
manufactured in 1968, the design is more reminiscent of machines designed
in the mid-1960's. My guess (and it is just a guess at this point) is that
Olympia introduced this calculator technology in Europe in the 1966
timeframe.

The next generation of calculators
from Monroe, such as the Monroe 990 switched
to the emerging small-scale integrated circuit technology, and abandoned
rather expensive magnetic core in favor of an acoustic delay-line for
working register storage.

This machine is in wonderful cosmetic and
functional condition, looking nearly new with the exception of a few small
scratches here and there, with very little signs of wear. Its condition
and functionality are testimony to the high quality of the design, as well
as the care that owners of the machine lavished upon it over the years.
I feel quite fortunate to have been able to add this wonderful example of
German engineering to the museum.

Model/Serial Number Tag

At the time this machine
was sold, the electronic calculator business was really starting to heat up.
Just about all the major forces in the electronics business were involved with
calculator technology by 1968. Monroe, a famous US maker of
wonderfully-designed mechanical calculators, was faced with a real problem
in the mid-1960's. Electronics were taking over, and the days of mechanical
calculators were seriously numbered. As a result of the pressure, Monroe
management decided to seek out electronic calculator makers outside the US.
These manufacturers in Europe (and later Japan) weren't selling in the
US marketplace, and Monroe could use them to design and build machines
which they could market in the United States to fend off the electronic
competition from other US makers like Friden and Wang, as well as the
aggressive Japanese makers Casio and Sharp. Early on, Monroe partnered
with the West German electronics manufacturer, Olympia A.G., to resell their
first-generation transistorized calculators into the US market under
the Monroe label. The Monroe 740 shown here is an example of this
relationship. Sadly, I haven't been able to find out the model
numbers of the corresponding Olympia machines. Along with the Model 740
exhibited here, it appears that Monroe also offered a model 730 (perhaps
with no memory functionality), and the Model 770
(which provided an additional memory storage register) that were 'clones'
of machines produced by Olympia. These three machines appear to be the only
desktop calculators sold by Monroe that were designed and manufactured by
Olympia. It seems that Monroe abandoned Olympia after these machines were no
longer viable in the marketplace, in favor of Japanese manufacturer Canon,
and later, calculators designed by California-based
Computer Design Corporation.

Display with Negative(Yellow) and Overflow(Red) Indicators.

The Monroe 740 is a basic four function machine, providing addition,
subtraction, multiplication and division. Unusual for the time is
the fact that the machine operates algebraicly, with an "=" key
used to calculate the result of a given operation. Many calculators
of the time use arithmetic logic. The calculator has two memory registers,
one that can serve as a general purpose accumulator (with add and subtract
functions), and a second memory register that is store/recall only.
The 740 operates with fully floating decimal point, which is quite unusual
for the time, since floating decimal point logic is more complex
than that required for fixed decimal operation. The display
of the 740 uses Nixie tubes, with fifteen tubes making up the display
panel. The Nixie tubes contain the digits zero through nine only. Decimal
points are made up of small discrete neon tubes that are positioned
between the Nixie tubes. The machine does not provide leading nor trailing
zero suppression, leaving it to the user to ignore insignificant zeroes.
Three incandescent status indicators are positioned behind yellow, red, and
green jewels on the front panel of the calculator. The yellow indicator
lights when the number on the display is negative. The red indicator
indicates an overflow condition, and the green indicator lights to show
that the memory register has non-zero content.

Monroe 740 with Covers Removed

Bottom View Minus Cabinet (Note Tank-Like Mechanical Structure)

The electronic brains of the Monroe 740
consist of a total of fourteen circuit boards (approximately
11" wide and 3 1/2" high) crammed with discrete transistors, diodes,
resistors, and capacitors. The boards are positioned low in the chassis,
with card guides holding the circuit boards in place. Each printed circuit
board has traces on the back side, and components and jumper wires on the front side.

The Left-Side of the 740 Chassis, with Connectors and Wire Harness

The Right-Side of the 740 Chassis, with Display Subsystem Connectors

Each circuit board connects into the
others via pin-type connectors at each end of the board. One end of
each board plugs into a fixed set of connectors on the right side
of the machine, and a removable connector plugs into the left end of
each board. The connectors are wired together with a maze of wire
arranged in an extremely nicely bundled harness.

The chassis of the machine is built
like a tank, composed of heavy-gauge stamped sheet metal, with lots of threaded
holes and machine screws holding everything together. The quality of
the hardware used to put the chassis together is impeccable, reminding me
of the insides of a Curta mechanical calculator
than that of an electronic instrument.

An Example of "Modular" Flip Flops

An interesting twist of the electronic design is that it appears that
standardized flip-flop modules are used. A small (approx. 1" x 1 1/2")
circuit board, with two transistors, and an assortment of resistors, diodes,
and capacitors makes up a module that serves as a flip flop. These
modules connect to the main circuit board by solid wire leads.
It appears that most of the flip flops used in the sequential logic of the
machine are built this way, with the remainder of the logic (traditional
diode-resistor gating, and transistor buffers and inverters) populated
directly on the circuit boards.

A View of the Monroe 740's Core Memory Plane

Core memory is used in the 740 for storage of the machine's working
registers. The core stack is quite primitive, with physically large
cores. The core plane appears to have been hand-wired - a tedious operation
that had to be done by people with extreme levels of patience. Each
core has four wires that pass through it, the "X", "Y", sense, and
inhibit wires. Given the age of the machine, and the fact that it operates
perfectly, I've been reluctant to disassemble the machine enough to
take boards out to get detailed photos and document the boards. For
this reason, I'm not entirely sure of the size of the core array.

Power Supply and Cooling Fan Detail

The electronics of the machine are
quite densely packed, and given the sheer number of components, even though
solid-state, quite a bit of heat is generated. The solution to potential
problems with overheating was to add a rather large "squirrel-cage" type
fan that pulls through vents in the bottom of the cabinet, up through the
circuit card cage, and out vent holes on the back panel of the cabinet.
The fan is surprisingly quiet, given that it operates at a relatively
low RPM, making up for lack of speed with sheer size. Along with the
fan and display assembly, the top part of the chassis is filled with
power supply electronics. The power supply is a traditional transformer-based
linear supply with transistor regulation. As with the rest of the machine,
the power supply appears to have been over-designed, with large, heavy
gauge heatsinks and components with lots of specification headroom.

Monroe 740 Keyboard Detail

The 740 uses a very intricate mechanical keyboard design. Each
key has a complicated arrangement of levers and bars that create a
mechanical interlock that prevents more than one key from being
depressed at a time. The lever arrangements actuate a microswitch for
each key on the keyboard, with the individual switches wired back to the
logic through a couple of pin-type connectors. The keyboard layout is
fairly conventional, with the left group of switches controlling memory
functions, decimal point positioning/rounding, and clearing the machine.
The center cluster of switches is a standard 10-key numeric entry pad, with
digits zero through nine and a decimal point. The rightmost group of
keys control the math and memory accumulator functions. The keycaps
themselves are made of a high quality plastic, and appear to have the
nomenclature moulded into them. The keys themselves show virtually no
wear, and the keyboard mechanism operates smoothly and quietly (as
mechanical keyboards go).

Store to Memory Register 1

Recall Memory Register 1

Recall and Clear Memory Register 2

Recall Memory Register 2

Display Memory Register 2

Shift Decimal Right and Round

Add to Memory Register 2

Subtract from Memory Register 2

Display Dividend

Divide

Keycap Decoding

Keycap nomenclature on the machine is
rather unconventional. Once its known what the functions of each key are,
the keycap legends make sense, but it took a little while to figure out
just what the keycaps meant. At first glance, it appears that the machine
is missing a divide key. However, one must remember that this machine was
built in Europe, where division is typically rendered as a ratio, for example,
1:2 indicating 1 divided by 2. So, the divide key is rendered as a ":".
The rest of the math function keys are labeled as expected. The other cryptic
keys on the keyboard are related to the memory functions of the machine.
The calculator has two memory registers, one which is usable only for storing
and recalling a single number, and the other as an accumulator that can
have numbers added to, or subtracted from. The store/recall register has
two keys that control it, one to store the number in the display into
the register, and another to recall it to the display. The other memory
register has four control keys. All of the control keys for this
register have a circle around the function legend. Two keys allow the
number in the display to be added to or subtracted from the register.
Two more keys recall the register to the display, one destructively, and
the other non-destructively. A fifth function key for the memory
register allows the memory register to be viewed without calling it into
the display. This key has an upward facing arrow with a circle around
it. When this key is depressed and held, the display temporarily switches
to show the content of the memory register. Once released, the display
reverts back to the previously displayed number. A key with a similar
legend, without the surrounding circle, works similarly, but temporarily
displays the dividend in division operations. The utility of this
function isn't really clear, but that's what it seems to do.
The last mysterious key is a key with a rightward-facing arrow, with
a semi circle underneath. This key is used to reposition the decimal
point one position to the right, while rounding off the number using
a '5-up, 4-down' rounding algorithm. This function is useful for
forcing results to a given number of decimal points. For example, a
retail merchant wants to decide the retail price of an item
that he sells. He purchased a lot of 200 of the items for a total
of $199 at wholesale, and he marks up the items by 2/3rds
for his profit. This means that the total sales price for all 200 units
should be $331.666666. Dividing this by 200 results in a sales price
per unit of $1.6583333. The merchant would press the "Shift/Round" key
five times to result in a final sales price of $1.66. Here's how
the Monroe 740 would display the results after each depression of the
"Shift/Round" key: 1.658333; 1.65833; 1.6583; 1.658; 1.66. Further
depressions would yield: 1.7, and finally 2. Continuing to depress
the "Shift/Round" key would result in the decimal point wrapping around
the left end of the display -- it appears no check is done to prevent
the user from shifting the decimal point 'too far'.

The 740 is relatively robust in its
detection of overflow conditions. When the machine is commanded to
perform an operation which exceeds its capacity, it lights up the
overflow indicator as soon as the overflow condition occurs, but
continues the calculation until complete, displaying an answer that
may not make much sense. Overflow conditions do not 'lock up' the
machine like many other machines. The overflow indicator stays on
until the user clears the machine with the "C" key. The machine does
not properly error out when commanded to divide by zero, however. Rather
it goes into 'churn' mode, with nothing but a bunch of zeros on the
display. Pressing keys while the machine is in this state will cause
apparently random and unpredictable results, ranging from flickering
decimal points to random numbers on the display and sometimes an
overflow condition. Pressing "C" when the machine is in 'divide by
zero madness' state clears up the condition most (but not all) of the time.
Sometimes the machine gets so confused that even pressing Clear won't
do the trick, requiring that the machine be powered off and powered back
on again before it will function properly. As with most machines with
core memory, the content of memory registers is retained while the machine
is powered off.

With regard to calculating speed, the 740 is no speed demon. Addition
and subtractions return results after a slight but discernable flicker
of the display, perhaps 80 to 100 milliseconds. Multiplication and
division return answers in a period of time determined by the complexity
of the operation, with 14 9's divided by 1 taking nearly one second
to complete. Trying the same operation with 15 9's results in an
overflow and incorrect answer -- a symptom that is fairly common
on early electronic calculators. Multiplication of 9999999 by itself
results in an answer in about 3/4 second. During the time calculations
are taking place, the display shows all zeroes, with a very slight
flickering of the uppermost few digits of the display.